The coupling coefficient Km represents the selectivity of signals in the frequency domain. The lower the coupling coefficient of a circuit, the higher the circuit's selectivity of signals.
In the design of a band pass filter using an inductor coupling resonant circuit, the coupling is usually realized with neighboring and parallel inductors. Therefore, it is necessary to have a larger space in order to obtain a smaller coupling coefficient. However, as the filter is miniaturized, the dielectric coupling in the circuits is likely to introduce additional noises. It is difficult to design a high-selectivity (narrow band) band pass filter. Furthermore, the coupling coefficients of the conventional filters are solely determined by the coupling inductance of parallel inductors. When designing miniaturized filters, the coupling inductance of parallel inductors will increase as a result of circuits getting to close.
FIG. 1 of the attached drawings shows the structure of a conventional symmetric band pass filter. The left part of the filter is a resonator, comprising a resonant capacitor 11 and a resonant inductor 12. The right part of the filter is also a resonator, comprising a resonant capacitor 14 and a resonant inductor 13. Resonant inductors 12, 13 form the coupling, having a coupling inductance Lm 16. Input signals enter the filter from the signal input end 10, and the signal output end 15 is for signal output. Resonant capacitors 11, 14 have the same capacitance, and resonant inductors 12, 13 have the same inductance.
However, as the coupling between the two resonant inductors 12, 13 is through the dielectric, the distance between the two inductors must be large enough in order to obtain a small enough coupling coefficient. On the other hand, as the distance between the two resonant inductors increases, the dielectric may incorporate more noises, which may greatly affect the characteristics of the filter.
FIG. 2 shows an equivalent circuit of a conventional symmetric band pass filter. The input signals enter from the signal input end 10, and the signal output end 15 is for signal output. The two inductors 22, 24 of the filter are in serial, and a parallel inductor 23 is added at the serial junction and connected to ground 17. A resonant capacitor 21 is place between signal input end 10 and ground 17, while another resonant capacitor 25 is placed between signal output end 15 and ground 17. Resonant capacitors 21, 25 have the same capacitance, and resonant inductors 22, 24 have the same inductance.
In the circuit of FIG. 2, the coupling coefficient Km of the filter is the value of the inductance of parallel inductor 23 divided by the sum of the inductance of serial inductor 22 and the inductance of parallel inductor 23. Therefore, to obtain a smaller coupling coefficient, it is necessary either serial inductor 22 having a very large inductance, or parallel inductor 23 having a very small inductance. However, in practice, serial inductors 22, 24 having large inductance are prone to generate more parasitic serial resistance, which decrease the signal level of the selected frequency. On the other hand, parallel inductor 23 having small inductance faces the following problem: as the inductance from the serial junction point of the two serial inductors 22, 24 to ground 17 is determined by parallel inductor 23 and the parasitic inductor from parallel inductor 23 to ground 17, it is necessary to minimize the inductance of parallel inductor 23 to obtain a smaller coupling coefficient when the parasitic inductance is relatively large. Due to the miniaturization, the components that should be independent are even more closely coupled. Therefore, it is practically difficult to minimize the parallel inductance under this structure.
It is obvious that the conventional inductor coupling resonant band pass filter is not suitable for miniaturization. Therefore, the present invention provides a π-type band pass filter to meet the miniaturization requirement.